Traditionally, a temperature sensor converts a temperature under test into a voltage or current signal, and then the signal is converted into a digital output through a analog-to-digital converter (ADC) thereof. In order to achieve better accuracy and resolution, it usually requires a bulky ADC which consumes high power and large chip size.
Moreover, with the scaling down of manufacturing processes as well as the decrease of operating voltage, high accuracy ADC becomes more and more difficult to design. Therefore such temperature sensor will become increasingly hard to manufacture.
Accordingly, a Taiwan Patent No. 1294029 proposes a time-domain smart temperature sensor, which converts a temperature under test into a clock signal, in which the period of the clock signal varies as the test temperature changes, and then the signal is converted into a digital output through a time-to-digital converter. Because the structure thereof is simple, the chip area and power consumption can be effectively reduced. The temperature sensor can also be modified to be full digital and can be even realized in a Field Programmable Gate Array (FPGA).
However, due to process variation, the digital outputs of said temperature sensors are usually different and a calibration has to be performed before each temperature sensor being used. Generally, a two-point calibration is used in the time-domain smart temperature sensor, and said two-point calibration is to measure two corresponding output values at two different calibration temperatures and then to calculate a first-order approximation line according to said two output values. Therefore, the digital output measured from each temperature sensor needs to apply its own approximation line, so as to be converted to an actual test temperature. For the above reasons, the conventional temperature sensor using two-point calibration needs to be calibrated at two different temperatures. Comparing to single-point calibration, the calibration cost is effectively doubled. Moreover, such conventional temperature sensor requires more memory to store the calibration data. It makes said temperature sensor less competitive in the market.
Although a recent paper “IEEE ISSCC Dig., February 2009, pp. 68-69” disclosed a time-domain smart temperature sensor for supporting a single-point calibration, it requires a complex dual delay-locked loop which makes not only the chip area larger but also the accuracy poorer.
Accordingly, there is an urgent need to improve the conventional technology to overcome the drawback in the prior art.